A marketing firm named App Promo has released some data on a developer survey it took, asking over 100 developers about how they were doing with selling and marketing their apps. Of those developers asked, over 59% told App Promo that they weren't even breaking even on the App Store. And a full 80 developers said they weren't making enough from App Store earnings to "support a standalone business."
App Promo also discovered (though it's not quite as surprising, given that this survey was done by a marketing firm) that the biggest earners they talked to spent almost $30,000 on marketing for their apps, and dedicated up to 14% of their time promoting and selling the app itself. That makes sense -- apps with bigger budgets and marketing money are more likely to be seen in the busy App Store listings.
Outside of App Promo's promotion here, that 59% stat means a lot. There is a lot of action on the App Store, it's true. But it's becoming harder and harder for individual developers to find a profitable place on Apple's platform.